WHAT'S THE BIG IDEA?, by George Lois (Doubleday: $22.50; 275 pp.). If you don't know who George Lois is, you clearly ought to: He seems to have stepped in just after God finished with the basics of creation, and created everything that has come since. Lois is in advertising--or, to accurately convey the tone of this combined memoir, self-promotion, how-to book, Lois is advertising--an art director cum entrepreneur who can lay claim to two of the most prestigious awards in the field, membership in the Art Directors Hall of Fame and the Creative Hall of Fame. It was logical enough to ask him to write about the field for advertising wanna-be's, but Lois Unleashed believes in teaching by example, so he dishes up one self-serving anecdote after another, most of them having to do with how he, the smartest ad man around, dealt with one dumb client after another. The trade press already has taken him to task for claiming authorship of the "I Want My MTV" line, which he says was an inspired update of his earlier "I Want My Maypo" line, even though other people swear they were the ones who dreamed it up. One wonders why an industry survivor such as Lois feels compelled to toot his own horn quite so stridently. It's too bad. When he quiets down and talks about the business, he's an astute historian.